The invention relates to a method for etching a substrate by means of a first etching mask, which comprises a first photoresist layer, with the etching process being interrupted before the first etching mask is applied, and the first etching mask being renewed by a second etching mask. The invention furthermore relates in general form to a method for producing an etching mask for etching a substrate.
The etching of a substrate, in particular the fabrication of deep trenches in a substrate, for example in silicon, is becoming ever more demanding with regard to the substrate etching, for example silicon etching, as the aspect ratio increases. Complex integration schemes are required for this purpose, which always have to provide a sufficiently thick etching mask. That is to say, the etching mask must be chosen such that it has not yet been completely consumed after production of the desired structure. The mask is generally a hard mask and must be deposited and opened in an appropriate manner. The opening process must in its own right in turn be carried out with an additional mask. All of this results in considerable complexity.
An oxide mask is generally used for silicon etching, and can in turn be opened using a silicon mask. That combination can be continued, but the resulting costs are very high.
It is accordingly an object of the invention to provide an improved method for producing an etching mask for etching a substrate, and, furthermore, of specifying an improved method for etching a substrate with a mask combination which can be continued, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows substrate etching to be carried out more cost-effectively.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for etching a substrate, which comprises:
etching the substrate with a first etching mask;
producing a second etching mask for renewing the first etching mask by
applying a second photoresist layer on the first etching mask or instead of the first etching mask and a reflective layer underneath corresponding to the first etching mask; and
illuminating the second photoresist layer such that areas that have been exposed once are not completely exposed, and areas that have been additionally exposed due to the reflective layer located under the second photoresist layer are exposed completely.
With the above and other objects in view there is also provided, in accordance with the invention, a method of producing an etching mask for etching a substrate, which comprises exposing a photoresist layer such that areas that have been exposed once are not yet completely exposed and that areas that have been additionally exposed due to a reflective layer located under the photoresist layer, are completely exposed.
The novel method is especially suitable in an etching process in which at least one layer that is deposited over a trench is etched to make contact with an opening of the trench, wherein the reflective layer is applied as a cover for the trench in a layer disposed under the at least one layer.
The method is also suitable, in a preferred implementation, for renewing an etching mask.
In summary, the objects of the invention are achieved by the above-outlined method for producing an etching mask for etching a substrate and, furthermore, by a method of the generic type for etching a substrate.
According to the invention, in order to etch a substrate, a first etching mask which comprises a first photoresist layer is used after a first, advantageously short, structuring of the substrate together with a reflective substrate, in order repeatedly to renew the etching mask for the substrate by means of a second photoresist layer. The use of a reflective substrate means that only the first etching mask need be structured, for example with a reticle, after which only a single, preferably homogenous, exposure with preferably parallel light is carried out in order to renew the first etching mask by means of the second photoresist layer, since the photoresist of the second photoresist layer has the characteristic of not yet being sufficiently exposed when illuminated once with the corresponding light, but of being exposed by additional illumination on the basis of the light which is reflected by the reflective layer. A negative photoresist is preferably used for the second photoresist layer. In this case, the renewed etching mask corresponds, after being developed, 1:1 to the originally used etching mask.
As specified in a further independent claim, the basic idea on which the invention is based, of producing an etching mask on the basis of information which is available in or on the substrate to be etched, in this case an appropriately structured reflective layer, can also be used in a general form, without the restriction to renewing a first etching mask.
In this casexe2x80x94as implemented in a specific embodiment, structures which are buried in the substrate can advantageously be exposed again 1:1 onto the substrate surface, without any need for complicated optical alignment of a reticle. A positive photoresist is advantageously used for this purpose, whose area which is exposed twice and which corresponds to the buried structure provided with the reflective layer is removed in a subsequent development process, as a result of which an etching mask which is suitable for reproduction of the buried structure and which corresponds to the etching mask used to produce the buried structure is produced in a simple manner.
In comparison with the prior art, the invention achieves, in particular, the advantages that even deep etching depths can be achieved for deep etching processes without the use of expensive hard masks, the use of which is in some cases problematic, since the etching mask can be renewed as often as required, and structures which are buried in the substrate can be imaged 1:1 onto the substrate surface, for example the wafer surface.
Owing to the capability to xe2x80x9cstorexe2x80x9d the information which is required for renewing or reproduction of the etching mask by means of a reflective layer when the etching mask is first used, that is to say during the first structuring of the substrate, by the previous structuring of the reflective layer, and to produce a reflective layer in a structure before, for example, epitaxial silicon is grown on it, the use of appropriate photoresist provides a very simple and cost-effective capability to avoid complex renewing and reproduction, which are subject to errors, of an etching mask with reticles.
With the above and other objects in view there is thus provided, in accordance with the invention, a method for etching a substrate, which comprises:
a) applying a reflective layer to the substrate;
b) applying a first photoresist layer to the reflective layer;
c) producing an etching pattern on the first photoresist layer and developing the first photoresist layer to form the first etching mask;
d) etching the reflective layer with the first etching mask;
e) etching the substrate with the first etching mask;
f) interrupting the etching process before the first etching mask formed by the first photoresist layer or before the reflective layer is consumed;
g) applying a second photoresist layer;
h) exposing the substrate coated with the second photoresist layer, such that areas which have been exposed once are not yet completely exposed and areas of the second photoresist layer which have been additionally exposed due to the reflective layer are exposed completely;
developing the second photoresist layer to form a second etching mask corresponding to the first etching mask; and
repeating the sequence, starting with the etching of the coated substrate but with the second etching mask, until a desired structure depth is achieved in the substrate.
In other words, after the final developing step the sequence is repeated, starting with the etching of the coated substrate but now using the second etching mask, that is to say starting with the step e), until a desired structure depth is achieved in the substrate.
If the etching process which is provided for the first structuring or at least for structuring of the reflective layer is interrupted before the first etching mask, which is formed by the first photoresist layer, is applied, it can also be removed before application of the second photoresist layer.
A protective layer is advantageously arranged between the reflective layer and the substrate to be etched, which protective layer in the case of a silicone substrate advantageously comprises a pad silicon dioxide layer which is applied directly to the silicon substrate, and a pad silicon nitride layer which is applied to the pad silicon dioxide layer and on which the reflective layer is applied. This protective layer is not necessary for the renewing or production of an etching mask according to the invention.
Furthermore, the photoresist which is used at least for the second photoresist layer advantageously does not have high absorption, in order that the difference between the single illumination process and the additional illumination process is as great as possible.
In accordance with an added feature of the invention, the reflective layer is composed of metal.
The exposure process is advantageously carried out according to the invention over the entire surface, homogeneously, using parallel light.
As stated above, a negative photoresist is advantageously used as a second photoresist for renewing a first etching mask, whose areas which have been exposed twice are used as a physical etching mask, that is to say they remain in place, after a development process.
If an etching mask composed of a positive photoresist is produced by means of the reflective layer in the above methods, then this etching mask has a structure which is the inverse of that of the reflective layer. This etching mask can be used, for example, to remove the reflective layer and, if appropriate, the protective layer located underneath it.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing and/or renewing and etching mask, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
FIG. 1-FIG. 7 show various stages in the method according to the invention for etching of a substrate, with a first etching mask being renewed by a second etching mask; and
FIG. 8 shows an outline illustration to explain the method according to the invention, in order to expose structures which are buried in the substrate 1:1 again onto the substrate surface.